In recent years, liquid crystal display devices, whose display characteristics have improved, are more and more used in television receivers, or the like. Although viewing angle characteristics of liquid crystal display devices have improved, there is a demand for further improvements. Particularly, there is a strong demand for improving viewing angle characteristics of liquid crystal display devices using a liquid crystal layer of a vertical alignment type (referred to also as liquid crystal display devices of a VA mode).
At present, liquid crystal display device of a VA mode, which are used in large-size liquid crystal display devices such as televisions, employ an alignment-divided structure in which a plurality of liquid crystal domains are formed in one pixel in order to improve the viewing angle characteristic. A mainstream method for forming an alignment-divided structure is the MVA mode. The MVA mode is disclosed in Patent Document No. 1, for example.
In the MVA mode, by providing an alignment regulating structure on the liquid crystal layer side of each of a pair of substrates that are opposing each other with a vertical alignment-type liquid crystal layer interposed therebetween, there are formed a plurality of liquid crystal domains of different orientation directions (tilt directions) (typically of four different orientation directions) within each pixel. The alignment regulating structure may be slits (openings) or ribs (projecting structures) provided on electrodes, exerting alignment regulating forces from both sides of the liquid crystal layer.
Using slits and ribs, however, the alignment regulating force acting on liquid crystal molecules becomes non-uniform, thereby resulting in a distribution of response speed, because slits or ribs are linear, as opposed to cases where the pre-tilt direction is defined by an alignment film as in the conventional TN mode. Moreover, the display luminance lowers because the light transmittance lowers in areas where slits or ribs are provided.
In order to avoid such a problem, it is preferred to form an alignment-divided structure by defining the pre-tilt direction using an alignment film, also with liquid crystal display devices of a VA mode. A liquid crystal display device of the VA mode, in which an alignment-divided structure is formed as described above, has been proposed in Patent Document No. 2 by the present applicant.
In the liquid crystal display device disclosed in Patent Document No. 2, the pre-tilt direction is defined by an alignment film, thereby forming a 4-divided alignment structure. That is, in the presence of a voltage applied across the liquid crystal layer, there are formed four liquid crystal domains within one pixel. Such a 4-divided alignment structure may also be referred to simply as a 4D structure.
In the liquid crystal display device disclosed in Patent Document No. 2, the pre-tilt direction defined by one of a pair of alignment films that are opposing each other with a liquid crystal layer interposed therebetween is generally 90° apart from that of the other alignment film. Therefore, the liquid crystal molecules assume a twisted orientation in the presence of an applied voltage. The VA mode, in which the liquid crystal molecules assume a twisted orientation due to the use of a pair of vertical alignment films provided so that their pre-tilt directions (alignment treatment directions) are orthogonal to each other as described above, may be referred to also as the VATN (Vertical Alignment Twisted Nematic) mode or the RTN (Reverse Twisted Nematic) mode. As already described above, since a 4D structure is formed by the liquid crystal display device of Patent Document No. 2, the present applicant refers to the display mode of the liquid crystal display device of Patent Document No. 2 as the 4D-RTN mode.
As a specific method for allowing the pre-tilt direction of the liquid crystal molecules to be defined by an alignment film, methods in which a photo-alignment treatment is performed as described in Patent Document No. 2 have been considered promising. Since the photo-alignment treatment can be done with no direct contact, no static electricity will occur due to friction as in a rubbing treatment, and it is possible to improve the production yield. Patent Document No. 3 also discloses a liquid crystal display device of the VATN mode using an alignment film (photo-alignment film) having been subjected to a photo-alignment treatment.